1. Field of the Invention
The present invention relates to pyrolysis naphtha treating methods for reduction of olefinic and diolefinic unsaturation without a concomitant conversion of aromatic hydrocarbons. More particularly, the present invention relates to a process for treating pyrolysis naphtha in the presence of a residuum hydrocarbon fraction under delayed coking conditions for reduction of reactive olefinic and diolefinic unsaturation and recovery of aromatic components of said pyrolysis naphtha.
Pyrolysis naphtha, such as that obtained as byproducts from hydrocarbon cracking processes for production of ethylene, consists of a mixture of highly unsaturated hydrocarbons. Hydrocarbon species represented in pyrolysis naphtha include aromatic hydrocarbons, polynuclear aromatic hydrocarbons, olefin and diolefin substituted aromatic hydrocarbons, olefin hydrocarbons, diolefin hydrocarbons, etc. Commonly, such pyrolysis naphtha may comprise about 70-90 percent of greater hydrocarbons containing aromatic nuclei. Such pyrolysis naphtha has a very high octane blending value, and upon stabilization of reactive olefinic and diolefinic hydrocarbons therein to prevent gum and/or heavy polymer formation such pyrolysis naphtha is particularly useful as a gasoline blending component. Additionally, pyrolysis naphtha is a source of aromatic hydrocarbons which are valuable as solvents, chemical raw materials, etc.
2. Description of the Prior Art
Pyrolysis naphtha, because of its high content of reactive olefinic and diolefinic hydrocarbons, is a highly unstable material and upon exposure to elevated temperatures forms gums, heavy polymers and/or coke-like materials. Consequently, in processes of the prior art for treating such pyrolysis naphtha, particularly hydrotreating processes, process equipment and catalyst tend to become fouled with such gums, heavy polymers and coke. Additionally, gasolines blended employing untreated pyrolysis naphtha have high gum contents which cause engine fouling upon use as a motor fuel. Additionally, for processes wherein aromatics are to be extracted from pyrolysis naphtha, olefinic hydrocarbons interfere with selectivity of the extraction solvent. As a consequence of these fouling problems and solvent selectivity problems it has been found highly preferable in the prior art that pyrolysis naphtha be hydrotreated for saturation of reactive olefinic and diolefinic compounds prior to treatment in other refining processes. However, due to the gum and heavy polymer forming tendency of pyrolysis naphtha upon exposure to elevated temperatures, hydrotreating catalyst exposed to untreated pyrolysis naphtha become rapidly deactivated and fouled.
Processes for treating pyrolysis naphtha prior to hydrotreating or other refining processes are known in the prior art. For example, one process comprises contacting pyrolysis naphtha with clay absorbent for absorption of olefin and diolefin hydrocarbons. Although this process is effective for removal of such olefin and diolefin hydrocarbons, it has a disadvantage in that large amounts of clay absorbents must be either regenerated or disposed of. Furthermore, a substantial portion of the pyrolysis naphtha is lost by absorption into the clay.
Another process for treating pyrolysis naphtha is taught in U.S. Pat. No. 3,400,160; Eng et al (1968). In this treating process, pyrolysis naphtha is contacted with silicaalumina cracking catalyst for polymerization of diolefin hydrocarbons, and subsequently the pyrolysis naphtha-cracking catalyst mixture is passed into a catalytic cracking zone wherein the polymers formed are cracked into lower boiling hydrocarbons. While this process appears effective for reducing the diolefinic unsaturation of pyrolysis naphtha, it is a two-step process and the silica-alumina catalyst must be either regenerated or disposed of in some manner.
Another process for treating pyrolysis naphtha is disclosed in U.S. Pat. No. 3,788,979; Caflisch et al (1974). One embodiment of this process for treating pyrolysis naphtha comprises separating the pyrolysis naphtha into a light fraction and a heavy fraction; cracking the light fraction thus producing monomers corresponding to polymers present in the pyrolysis naphtha; hydrogenating said cracked light fraction for saturation of olefinic hydrocarbons; and recovering a high octane blending component from said hydrogenated product. Another embodiment of this invention comprises cracking full boiling range pyrolysis naphtha; fractionating the cracked product into a light fraction and a heavy fraction; hydrogenating the light fraction for saturation of olefinic hydrocarbons; and recovering a high octane blending component from said hydrogenated product. The first embodiment of the process of this particular patent requires fractionation of the unstable pyrolysis naphtha under conditions which may result in fouling of process equipment such as reboilers and heat exchangers. Additionally, a substantial amount of pyrolysis naphtha, represented by the heavy ends, remains untreated according to the process disclosed.